Cranes are used to raise, shift and lower heavy objects or loads by means or a projecting, swinging arm or boom in conjunction with a hoisting apparatus. The hoisting apparatus can comprise a cable extending adjacent the boom, wound around a sheave situated near the inner end of the boom at one end and having a load block assembly attached at the other end. The load block assembly can consist of an upper load block or boom point sheave assembly attached at the outer end of the boom or boom point and a lower load block or hook assembly, which includes a movable sheave and a load attachment means. The cable extends past the upper load block to support the lower load block. During the normal course of operation of the crane, the lower load block is maintained at a distance below the upper load block.
Such a crane can contain various moving parts which are coordinated to facilitate the attachment and transportation of objects. The boom can have the capacity for being elevated and lowered, as well as having extendable and retractable telescoping boom sections. Additionally, the cable can be reeled in and let out over the inner boom sheave. The retrieval and transportation of a load is achieved through various combinations of reeling in and paying out the cable, raising and lowering the boom, and extending and retracting the telescoping boom sections.
There is a danger, during the operation of the crane, of the lower load block coming in contact with the boom point, upper load block or other components attached thereon. Such contact could result in costly damage to the crane or its load. The cable could break resulting in the load and lower load block falling to the ground. The load could detach from the lower load block and fall. Alternatively, the load block could move over the boom point with the resulting interference damaging the components at their point of contact. Each possibility is clearly undesirable.
"Two-blocking" refers to the condition when the lower load block comes in contact with the upper load block. Extending or lowering the boom without sufficiently unwinding the cable and reeling in the cable too far can cause two blocking. As mentioned previously, it is important that the lower load block be prevented from coming into contact with the upper load block. Therefore, cranes installed with a load block assembly would benefit greatly from an anti-two block device.
There are safety control systems adapted for installation on extension booms that prevent two-blocking. In a typical system a switch is installed near the upper load block. The switch has a lever with a chain and a weight attached thereon. The cable is threaded through the weight. The weight maintains the switch in the working position. As the lower load block comes in contact with the weight, the tension in the chain relaxes, thereby releasing the lever of the switch which stops all activity which could endanger the boom apparatus.
In the past, various patents have issued relating to certain anti-two block systems.
For example, U.S. Pat. No. 3,922,789, issued on Dec. 2, 1975 to I. D. Sarrell, describes a boom length sensing system with a two-block condition sensing. This system includes the use of a single electrically conductive cable mounted on the boom of the crane. The cable is wound on a spring-biased reel mounted for rotation on a non-extending portion of the crane boom with one end of the cable secured to and electrically connected to the uppermost movable section of the crane boom. A constant current is supplied to the cable through an electrical contacter arranged to permit movement of the cable as the boom is varied in length. The constant current is also supplied to a circuit in parallel with the cable and the circuit senses a resistance value for the length of the cable between the contacter and the end secured to the movable section of the boom crane in response to the current flowing through the cable. The circuit in parallel with the cable senses an open condition of the normally closed switch so as to provide a manifestation of an impending two-block condition.
U.S. Pat. No. 4,280,628, issued on Jul. 28, 1981, to Goss et al. describes an anti-two block system which is adapted to be used with a crane for the prevention of damage to the boom tip of the boom of the crane by the travelling member. The anti-two block system includes an engaging mechanism movably mounted with the boom tip for engaging the travelling member when the travelling member moves into close proximity to the boom tip during lifting operations of the travelling member.
U.S. Pat. No. 4,523,686, issued on Jun. 18, 1985, to Moore et al., also describes an anti-two block system. This system includes a boom hoist drum driven by a first hydraulic motor with a first variable displacement motor controlled by a first ram movable in a given direction so as to supply fluid pressure for driving the boom drum to lower a boom and a hook drum driven by a second hydraulic motor with a second variable displacement pump controlled by a second ram movable in a given direction to supply fluid pressure for driving the hook drum to raise a hook block. A differential mechanism sums the movement of the hook block and boom and activates a switch to energize a solenoid valve. When the hook block is a predetermined distance from the boom, the differential mechanism serves to preclude movement of the rams in the given directions.
U.S. Pat. No. 5,263,660, issued on Nov. 23, 1993 to J. Brozik, teaches another anti-two blocking device for preventing a lower load block on a crane from coming into contact with an upper load block. The device includes a cable follower through which the crane's cable is threaded and situated between the upper load block and the lower load block. When the lower load block is within a predetermined distance of the upper load block, the lower load block reaches the cable follower and exerts an upward force which causes the cable follower to activate an actuator for a switch. The switch deactivates all activity on the crane which could cause the lower load block to come into contact with the upper load block.
It is an object of the present invention to provide an anti-two block system which prevents contact between the travelling member and the boom tip of the crane.
It is another object of the present invention to provide an anti-two block system that can be easily installed.
It is another object of the present invention to provide an anti-two block system which avoids contact with the hoist line of the crane.
It is another object of the present invention to provide an anti-two block system in which each of the components can be easily replaced at minimal cost.
It is another object of the present invention to provide an anti-two block system which minimizes electrical and mechanical components while increasing the life of the system.
It is still another object of the present invention to provide an anti-two block system which is relatively inexpensive, easy to manufacture, and easy to use.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.